Mosquito bump immunity?

I remember getting bumps as a small child, and I particular recal sleeping under the stars on a warm summer night in nothing but shorts, and the next morning being covered in mosquito bumps from head to toe, along with my brother. That, however is the last time I can remember having a bump or itch caused by a mosquito. That was over 20 years ago, and I am an avid camper and fisherman. They still bite me frequently, and they are still annoying, but I never get a bump or itch. How is this possible? My brother still gets itchy bumps. I figure I somehow gained immunity, but is that even possible?